The present invention relates generally to linear synchronous motors and more particularly to a linear synchronous motor having variable pole pitches generating propulsion and levitation forces for a high speed transportation system where the stator defines a guideway for the transportation vehicle.
The proposed linear synchronous motor represents an improvement over the linear synchronous motor having variable pole pitches that is described in U.S. patent application Ser. No. 691,430, the disclosure of which is incorporated herein by reference.
Because the stator of the linear synchronous motor utilizes a current of constant frequency, a change in the speed of the rotor is achieved by proportionally changing the length of the phase coils of the stator winding and the pole pitches of the rotor. In the process, the mean value of the induction in the air gap between the rotor and the stator does not change, but the area defined by the loops of the phase coils changes in direct proportion to their length. Therefore the magnetic flux traversing the phase coils of the stator's windings increases along the acceleration section of the guideway/stator in proportion to the increase in the vehicle's speed.
In a linear synchronous motor having variable pole pitches the lines of force of the magnetic field form a loop that closes between the adjacent poles of the rotor and which passes through the steel cores. Therefore, the total magnetic flux linked to each phase coil must flow through the cross-sectional area of each core. Hence, the magnetic flux passing through the cross section of the core increases as the speed of the vehicle increases. If the cores are manufactured with equal cross-sectional dimensions along the entire course of the guideway/stator, then within the core, the flux density (i.e., the magnetic induction in the steel core) will increase in the accelerating section, attaining a magnitude within the constant-speed section that exceeds the permissible saturation of the magnetic steel. Once saturation is reached, the magnetic resistance of the cores dramatically increases, the flux decreases, and the tractive and levitational forces of the linear motor decrease as well. To avoid these problems, the cross-sectional dimension of the stator cores must be increased in proportion to the increase in the speed of the vehicle along each section of the guideway/stator. Consequently, the quantity of steel required for constructing the cores is very high, as are the associated costs.
In the linear synchronous motor disclosed in the copending application, the levitational force is achieved by deforming the magnetic field in the air gap that is created by the rotor's permanent magnets. The rotor is displaced downward relative to the stator cores as a result of the weight of the vehicle. Consequently, there is a deformation of those portions of the uniform magnetic field in the air gap that are near the top and bottom edges of the permanent magnet. As a result, the tubes of the magnetic field (i.e., the region between parallel lines of the magnetic field) are stretched, and the magnetic lines of force are lengthened. The magnetic tubes, which characteristically attempt to minimize their length, create forces that have vertical components that attract the magnets to the steel cores. The vector sum of the attracting forces creates the levitating force of the motor.
As can be seen in FIG. 4B, corresponding to FIG. 8 of copending patent application Ser. No. 691,430, when the rotor is shifted downward, only a small portion of the uniform magnetic field is actually deformed. The magnetic field remains uniform along most of the length of the air gap and hence does not participate in the creation of a levitational force. Therefore, most of the energy from the rotor's permanent magnets is not effectively used to generate the levitational force of the motor.